The increasing use of natural gas as a fuel has resulted in a need to remove nitrogen from some natural gas sources, in order to meet Wobbe Index and calorific value specifications, particularly where the gas is delivered into a country's gas transmission system. The nitrogen may either be naturally occurring or resulting from nitrogen injection into oil fields for enhanced recovery.
A particular problem is to design a process for efficient removal of nitrogen from natural gas feed at high pressure (75 to 130 bar absolute; 7.5 to 13 MPa), with relatively small concentrations of nitrogen (5 to 15 mol %), and to produce sales gas at a pressure similar to the feed gas pressure.
A further problem is that, as gas reservoir pressure decays to below the required sales gas pressure (e.g., about 75 bar absolute (7.5 MPa) in the case of the United Kingdom's National Transmission System), feed gas compression needs to be added. This is a relatively expensive investment since it is not utilized fully throughout the life of the nitrogen removal unit (NRU).
Therefore, an object of the present invention is to provide an improved process to remove nitrogen from natural gas feed with low nitrogen content (5 to 15 mol %) and at high pressure (75 to 130 bar absolute; 7.5 to 13 MPa). It is a further object of this invention to provide a process for removal of nitrogen from natural gas feed, which is sufficiently flexible to operate at lower feed pressure (25 to 75 bar absolute; 2.5 to 7.5 MPa) while still producing sales gas at higher pressure (about 75 bar absolute; 7.5 MPa), without the need for feed gas compression.
Nitrogen removal from natural gas is usually most economically effected by cryogenic distillation. Numerous cycles have been developed, many based on the concept of double distillation columns as used in air separation. One problem associated with double column cycles is that, at feed nitrogen concentrations less than 25 mol %, the quantity of reflux liquid that can be generated is insufficient to achieve an economic recovery of methane. Another problem is that relatively low concentrations of carbon dioxide and hydrocarbons, such as benzene, hexane and heavier components, would freeze at the cryogenic temperatures associated with the lower pressure column.
GB-B-2,208,699 describes an improved process that is less energy intensive at low levels of feed nitrogen concentration, in which the separation is effected in two columns with integrated condensation of overhead first column vapor and second column reboil. While this process overcomes the problems mentioned above, it is relatively complicated and expensive.
U.S. Pat. No. 4,415,345 discloses the removal of nitrogen from a natural gas feed stream by a cryogenic process using primary and secondary distillation columns operating at different pressures. Primary column methane-rich bottoms liquid is cooled by heat exchange against secondary column bottoms liquid and secondary column nitrogen-rich nitrogen overhead and then expanded prior to feeding to the secondary column. Primary column nitrogen overhead provides reboil to the secondary column and is returned to the primary column and/or secondary column as reflux.